1. Field of the Invention
The present invention relates to a power conversion apparatus.
2. Description of the Related Art
FIG. 5 is a block diagram showing a circuit arrangement of a known power conversion apparatus for converting direct-current power supply into three-phase alternating current to drive an alternating-current load such as a three-phase alternating-current motor. In FIG. 5, reference numeral 1 designates a switching power module for performing power conversion by means of switching, numeral 7 denotes a battery constituting a direct-current power supply, and numeral 8 depicts an alternating-current load such as a three-phase alternating-current motor. The switching power module 1, for example in the case of an electric vehicle, converts a discharge output (direct current) of the direct-current power supply 7 into a three-phase alternating current for driving the alternating-current load 8 in starting or accelerating the vehicle. On the other hand, in regenerative braking, regenerative power from the alternating-current load 8 is converted from a three-phase alternating current into a direct current, and then returned to the direct-current power supply 7. In the interior of the switching power module 1, contained are switching devices 2 including transistors for power conversion from direct current into three-phase alternating current, IGBTs and MOSFETS, free wheeling diodes 3 for power conversion from three-phase alternating current into direct current, snubber capacitors 4 for restraining surge occurring in the switching device 2 sections at switching operations, and a control circuit section 5 for controlling the switching devices 2. In this case, the major requirement for the snubber capacitors 4 is an excellent frequency characteristic, with a film capacitor being common. Additionally, in FIG. 5, numeral 6 represents a smoothing capacitor for suppressing voltage variation of the direct-current power supply 7 at the switching to smooth the jumping or the like of the voltage. The smoothing capacitor 6 requires a sufficiently large electrostatic capacity; therefore, an aluminium electrolytic capacitor is commonly put to use because a large electrostatic capacity is easily obtainable. Incidentally, the control circuit 5 is a common circuit for controlling the alternating-current load 8 such as a three-phase alternating-current motor, and a detailed illustration thereof will be omitted here.
Furthermore, FIG. 6 is a side-elevational cross-sectional view showing a common internal configuration of a known power conversion apparatus. In FIG. 6, reference numeral 21 designates a case, which contains a packaged switching power module 1, smoothing capacitors 6 and a snubber capacitor substrate 18 holding snubber capacitors 4 (not shown). In general, a wiring (distributing) board 19 such as a copper bus bar or a copper plate is used for establishing the connection between the switching power module 1 and the smoothing capacitors 6, and fixing and electrical connection thereof are made through the use of screws. Additionally, in general, the snubber capacitor substrate 18 is located in the vicinity of positive-electrode (P) and negative-electrode (N) direct-current input wirings (wiring sections) 9p and 9n and a U-phase, V-phase and W-phase alternating-current output wiring (wiring section) 10 on the switching power module 1 and fixed and electrically connected through the use of screws 20 (in this case, exactly, the alternating-current output wiring 10 comprises three output wirings 10u, 10v and 10w corresponding respectively to the U-phase, V-phase and W-phase as shown in FIG. 5, but they are simplified in FIG. 6). Moreover, the package of the switching power module 1 is composed of a resin-made switching power module case 12 accommodating the positive-electrode (P) and negative-electrode (N) direct-current input wirings 9p, 9n, the U-phase, V-phase and W-phase alternating-current output wirings 10u, 10v and 10w and a control circuit substrate connection wiring 11 in an insert molding manner, and a switching power module base plate 13. Still additionally, the package of the switching power module 1 accommodates an insulating substrate 14 such as a ceramic substrate for holding the switching devices 2 and the free wheeling diodes 3, and a control circuit substrate 17 for supporting the control circuit section 5. The switching devices 2 and the free wheeling diodes 3 are fixedly secured on the switching power module base plate 13 through the insulating substrate 14 with an electrically conductive pattern by means of an adhesive material such as a solder, and further connected to the positive-electrode (P) and negative-electrode (N) direct-current input wirings 9p, 9n, the U-phase, V-phase and W-phase alternating-current output wirings 10u, 10v, 10w and the control circuit substrate connection wiring 11 through the use of a connection conductor 15 such as wire bonding. Furthermore, the control circuit substrate 17 is connected electrically to the control circuit substrate connection wiring 11 by means of soldering or the like. Still furthermore, a gel-like filler 16 is put between the insulating substrate 14 and the control circuit substrate 17, and if necessary, a resin such as epoxy can be placed thereon. The gel-like filler 16 is for protecting the switching devices 2, the free wheeling diodes 3 and the connection conductor 15 to prevent the switching devices 2 from failure or malfunction due to humidity or dust. In addition, a cooling member 22, which cools the switching devices 2 using air, water, oil or the like is attached to the case 21 so that the Joule heat generated from the switching devices 2 is radiated through the insulating substrate 14 and the switching power module base plate 13 into the cooling member 22 for cooling the switching devices 2.
In the known power conversion apparatus thus constructed, the smoothing capacitors 6 are required to have an electrostatic capacity large sufficiently to smooth the output of the direct-current power supply 7 to be applied to the switching devices 2, thus their sizes increase generally. Although the smoothing capacitor 6 is made commonly using an aluminium electrolytic capacitor, since its internal resistance is high, the internal heat generation of the smoothing capacitor 6 increases due to the direct-current ripple voltage variation occurring at switching operations. For restraining this heat generation, there is a need to complicate the construction considerably for cooling the smoothing capacitor 6 through the use of the aforesaid cooling member 22 or to further increase its electrostatic capacity. For this reason, in the case of the known power conversion apparatus, the area and volume of the smoothing capacitor 6 enlarge, thus increasing the size of the entire apparatus.
In addition, since the aluminium electrolytic capacitor for use on the smoothing capacitor 6 has disadvantages in that its operating temperature range is narrow and its service time (namely, duration of life) is short from the influence of leakage of electrolyte caused by seal deterioration, thus lowering the reliability.
Still additionally, since the area and volume of the smoothing capacitor 6 are large, a wiring board 19 for the connection between the switching power module 1 and the smoothing capacitor 6 becomes long. Accordingly, the wiring inductance between the switching device 2 and the smoothing capacitor 6 increases, which produces the possibility of generating a large surge at switching to destroy the switching device 2. Thus, a need exists to install a snubber capacitor in the vicinity of the positive-electrode (P) and negative-electrode (N) direct-current input wirings 9p, 9n and the U-phase, V-phase and W-phase alternating-current output wiring 10 on the switching power module 1, which makes the manufacturing process troublesome and increases the cost.
Moreover, in the case of connecting the aluminium electrolytic capacitor to the switching power module connection wiring board 19, there is a need to screw the terminals of every positive electrode (P) and every negative electrode (N) of all of aluminium electrolytic capacitor, which makes fabrication difficult.